Machinability and microstructural studies on phase transformations in Austempered Ductile Iron
Austempered Ductile Iron (ADI) is a type of nodular, ductile cast iron subjected to heat treatments - austenitising and austempering. The heat treatment gives ADI its unique ausferrite microstructure through which ADI gets its advantageous material properties. Possibly the most significant hurdle for the engineering community to overcome, to fully realise the potential of ADI, is in its successful machining. Whilst machining is conducted prior to heat treatment and offers no significant difficulty, machining post heat treatment is demanding and often avoided. Phase transformation of retained austenite to martensite leading to poor machinability characteristics is a common problem experienced during machining.
This research was divided into two categories: characterisation of ADI and study of phase transformations. Machinability of grades 900, 1050, 1200 and 1400 was evaluated using surface texture, microhardness, chip morphology and metallography analysis. The assessment was conducted on a cross-sectioned hole surface obtained from drilling at a pre-defined set of cutting parameters - cutting speeds (rpm) of 697 and 929, feed rates (mm/rev) of 0.1 and 0.2, coolant on/off and hole depth, 25mm. Grade 1200 was the best rank grade exhibiting good machinability characteristics.
Study of phase transformations was an investigative study on the factors - plastic strain (εp) and thermal energy (Q) which effect phase transformations during machining. The experimental design consists of face milling grade 1200 at variable Depth of Cut (DoC) range from 1 to 4 mm, coolant on/off, at constant speed, 1992 rpm and feed rate, 0.1 mm/tooth. Plastic strain (εp) and martensite content (M) at fracture point for each grade was evaluated by tensile testing. The effect of thermal energy (Q) on phase transformations was also verified through temperature measurements at DoC 3 and 1 mm using thermocouples embedded into the workpiece. Finally, the amount of plastic strain (εp) and thermal energy (Q) responsible for a given martensite increase (M) during milling was related and calculated using a mathematical function, M=f(ε_p , Q).
Future work considers an in-depth study on the tool materials, geometry and wear; microstructural displacement during phase transformations using quick-stop (chip freezing), evaluating ADI compatibility for high speed machining and extending the experimental trials to grades 900, 1050 and 1400.